Composition and method for electrolytic stripping of coatings from metals



3,257,299 COMPOSITION AND METHOD FOR ELECTROLYTIC STRIPPING M. MEKJEAN June 21, 1966 OF COATINGS FROM METALS 3 Sheets-Sheet 1 Filed Sept. 26, 1961 June 21, 1966 MEKJEAN 3,257,299

COMPOSITION AND METHOD FOR ELECTROLYTIC STRIPPING 0F COATINGS FROM METALS Filed Sept. 26, 1961 5 Sheets-Sheet 2 June 21, 1966 M. MEKJEAN 3,257,299

COMPOSITION AND METHOD FOR ELECTROLYTIC STRIPPING OF COATINGS FROM METALS Filed Sept. 2e 1961 5 Sheets-Sheet a United States Patent COMPOSITION AND METHOD FOR ELECTROLYT-.

IC STRIPPING OF COATINGS FROM METALS Matthew Mekjean, Niagara Falls, N.Y., assignor to Hooker Chemical Corporation, Niagara Falls, N.Y., a corporation of New York Filed Sept. 26, 1961, Ser. No. 140,925 12 Claims. (Cl. 204-141) stripped of their surface coatings. This system generally comprises a metallic pot containing an electrolyte or molten bath composition. Immersed in this electrolyte, or in contact therewith are anode means and cathode means. The pot may act as the cathode means. The system may also comprise a consumable electrode to be used in the system as the anode or cathod means in lieu of the pot. By surface coating is meant any covering on the surface of a basis metal workpiece (of any size or shape), which colors, protects, beautifies, strengthens, increases wear resistance, reduces friction, increases the value and/ or utility of the workpiece. These coatings on basis metal workpieces are, in the broadest sense, com prised of any of the following:

(a) One or more metal coatings, produced by dipping or plating methods or vacuum techniques.

(b) Oxide coatings, such as anodizing of aluminum, blueing of steels, etc.

(c) Dyed oxide coatings, as the coloring and sealing of anodized aluminum.

(d) Paint, lacquer and enamels and other organic coatings (containing organic or inorganic pigments).

(e) Phosphate coatings.

(f) Chromate conversion coatings.

(g) Glass, porcelain and other silicate-type inorganic coatings.

(h) Rubber, clo h, plastics (other electrical insulating materials) coatings.

REASONS FOR STRIPPING (METALLIC) COATINGS FROM METALS (1) Salvage of the basis metal coated per se, which means that the object being stripped is re-usable or re- 3,257,299 Patented June 21, 1966 invention is almost universal in its scope in that all types of coatings on metallic workpieces can be removed expeditiously, rapidly and efficiently without (essentially) any deleterious effects on the basis metal whatsoever. The only limitation to the use of this method is that the basis metal must be substantially unreactive with the salt bath, and must have a melting point which would be above that of the operating temperature of the bath. The ensuing discussion will refer to the metallic workpieces as coated copper; however, it is understood that other metals (for example, steel or copper alloys), can

equally be stripped of their metallic coatings; the invention is in no way limited to the stripping of copper objects.

It is known in the prior art that metallic coated scrap copper can be treated'in aqueous media to remove the adhering metallic coating. These systems have been somewhat limited commercially because of economic pairable, and is probably intended for replating or r'ecoating and used again for the same purposefor which it was made originally. For example, in stripping residual chromium and/or copper from automobile bumpers or bumper-guards, so that they may be replated for reuse-extending the value, life and utility of the original parts. Example: Factory rejects of fabricated parts after coating, such as household appliances, auto parts, machine tool parts, etc. (2) Salvage of the coating material itself, e.g., as in the reclaiming of tin from tin cans, either in the form of tin salts or as metallic tin or reclaiming previous metal coatings from baser metal objects. (3) Salvageof the basis metal as scrap, in a rough form of refining. For example: in detinning of tin cans, the stripped steel cans are compacted in bales and returned to the steel mills as scrap; the dissolved tin is also eventually reclaimed as salts (sodium stannate), or is refined to tin itself. For example: in stripping tin, lead, solder, etc., from copper wire and heavy cables, for upgrading the quality of the reclaimed copper. If the size of the operation warrants the economics of the process, the stripped metal may also be recovered as a salt or as the metal.

The process, composition and technique used in this considerations and safety factors. Also, it has been found that lead and tin coatings are extremely difiicult to remove from copper base objects. Salvaged copper must essentially be as lead-free and tin-free as virgin copper for obtaining the highest resale price. Any process that approaches the optimum in salvaging substantially lead and/or tin-free copper will undoubltedly find large scale acceptance in the art. It was found that in molten bath systems copper base material was initially satisfactorily separated from its metallic coatings; however, the active life of the bath itself was short. After processing leadand tin-coated copper-based material, the salt was deactivated apparently due to an inhibiting effect of lead and/ or tin contamination. The problem faced therefore, was to develop a method of removing the tinand leadcoated surfaces from copper and at the same time providing a commercially useful system that would remain active after continued use. The molten alkali metal hydroxide baths were effective systems. However, the problem of extending the bath life was again encountered. It was surprisingly found that if fixed proportions of additives were added to the above-mentioned molten sodium hydroxide bath, the effectiveness of the bath was substantially improved. A further means was required however to retain this effectiveness over an extended period of time. It was discovered that while the first workpiece was somewhat stripped, the subsequent pieces were progressively inferior. Apparently, the lead coating removed formed a lead oxide sheath on the workpiece, thereby preventing the required contact of the workpiece surface with the molten bath; also lead oxide was formed in the bath proper, resulting in substantial deactivation of this same bath. Thus, what was essential to a successful process was an alkali metal hydroxide bath, the components of which could not be deactivated by lead or tin, or be slowed down as these metals tend to saturate the melt. It was discovered that by the electrolytic approach, the active (reducing) agents were supplied on demand by direct current, which represented an inexhaustible external source of energy. With this electrolytic action, the lead oxide ions tend to migrate to the anode in the system, and the surface of the workpiece is kept clear of lead oxide, thereby providing easy access for the stripping action of the bath. In this concept of electrolytic stripping (using the stripping of a tin surface coating as example), the workpiece is initially made (first step) anodic. By this step, an oxide coating is created on the surface of the plated metal. The anode reaction would generally be as follows: Sn+20= SnO +4e. In this reaction a thin layer of metal (tin) is converted to the oxide. The next (step 2) is to make the workpiece cathodic by reversing the current. In this step the oxide created in the first step is reduced to the metal (atomic) and oxygen gas. Cathode reaction: SnO +4e Sn+20=. Anode reaction: 20-4e 0 (gas). Steps 1 and 2 are repeated as often as required to remove the coating of interest.

The novel composition of the bath of this invention comprises an alkali metal hydroxide having added thereto an agent comprising any or all of the following: carbonates, halides, sulfates, phosphates, borates, stannates, zincates, plumbates, silicates, molybdates, fiuosilicates, titanates, zirconates, fluotitanates, fluozirconates, fluoborates, or mixtures thereof. The proportions of these components are at least fifty percent alkali metal hydroxide to one to fifty percent of the above mentioned agents. The preferred embodiment of this composition comprises about eighty to eightyefive percent technical grade NaOH, five percent of Na PO two to five percent Na SnO and ten percent potassium hydroxide. Other more specific baths preferred are:

Bath A: Percent by weight Sodium hydroxide 82 to 84 Sodium carbonate 0.5 to 1.5 Sodium chloride 0.5 to 1.5 Disodium hydrogen phosphate 5 Potassium chloride Bath B: Percent by weight Sodium hydroxide 82 to 84 Sodium carbonate 0.5 to 1.5 Sodium chloride 0.5 to 1.5 Trisodium phosphate 5 Potassium chloride 10 Bath C: Percent by weight Sodium hydroxide 82 to 84 Sodium carbonate 0.5 to 1.5 Sodium chloride 0.5 to 1.5 Monosodium hydrogen phosphate 5 Potassium hydroxide 10 In this invention the pot usually acts as anode, but not exclusively. There are occasions when the pot, acting as an electrode, is made either anode or cathode. In such a case, it is preferable to supply auxiliary consumable electrodes, usually in the form of grids to allow free circulation of electrolyte in the system, and the metallic workpieces act as the opposite electrode, either at any one moment, as cathode or anode means.

Standard prior art stripping methods involve aqueous chemical and electrochemical processes. Aqueous chemical acid systems for dissolving these coatings also attack the base metal-metal losses are excessive, process rates of removal are relatively slow, base metal surfaces are etched and pitted, and costs make the process uneconomical. The evolution of hydrogen, acid mists, nitrous oxide, etc., also introduce safety hazards to operating personnel, and such equipment requires extensive and costly ventilation.

Oxidizing caustic soda aqueous systems for removal of tin coatings from steel, as in stripping of tin cans, is straightforward and well-known in the art. Complications arise, however, in other systems. In removing tin from copper or copper-base alloys or from steels, or lead coatings from copper-base alloys, or zinc galvanizing from steel or aluminum coatings from steel or from copper base alloys, or of chromium plates from steel or copper-based alloys, or copper plating on steel, or silver plates from copper or copper-base alloys, or any metal plate or coating from any other metal as a base; these all require special stripping techniques, special compositions of matter, and specific processes.

The other method in aqueous stripping is electrolytic stripping, in which the workpiece is the anode. Electrolytic solutions vary in life and in stripping rate. The basic principle is a form of reverse plating in which the part to be stripped is the anode, losing its plate to the cathode, where it is replated and/ or reclaimed. Most electrolytic stripping electrolytes present problems of stripping rates and voltages at which the basis metal will not dissolve or become pitted. The best of these electrolytes are cyanide solutions because of high electrical efficiency, but they are toxic. However, because of the relatively slow rates of stripping, and complications arising out of variations in the alloy composition of the basis metal, each application must be evaluated on its own merits. Specific voltages, current densities, temperatures, solution concentrations, pH, as well as a few other minor factors, must be predetermined experimentally. Only the more precious metal can successfully absorb the cost of these processes.

Many variables confront the commercial stripper. A process that works on thin deposits, fails when the deposits are heavier. It may be discovered that it is relatively simple to strip a freshly deposited metal; but impossible to remove an old deposit. Substituting glycerine, alcohols or sugars for aportion of the water present may be found to be necessary to energize the activity of stripping, while at the same time deactivating the tendency of the basis metal to dissolve and pit. At too low a voltage, stripping may safely proceed without attack on the basis metal, but the stripping rate may prove uneconomical; at higher voltages, basis metals may be attacked, or polarization may occur with a decrease in the stripping rate. In other words, experience in the art, plus generous experimentation will define a successful commercial stripping concern. There is little, if any, predictable process that can be based exclusively on straight facts and scientific principles in dealing with a specific case in point.

The following drawing further illustrates the particulars of this invention.

FIGURE I is a diagramatic View illustrating the system of this invention.

FIGURE 11 is a diagramatic view illustrating an alternate concept of the system of this invention.

FIGURE III is the same as FIGURE II wherein the current reversal is illustrated.

In FIGURE I pot 1 preferably constructed of steel, contains a molten bath composition 2'. The composition comprises an alkali-metal hydroxide having added thereto a non-oxidizing agent selected from the group consisting of carbonates, halides, sulfates, phosphates, borates, 'stannates, zincates, plumbates, silicates, molybdates, fiuosilicates, fiuoborates, or mixtures thereof. The preferred composition of this invention comprises about eighty-five percent NaOI-I, about five percent Na PO and about ten percent KCl. Immersed in said composition is a metal workpiece 3 (or a steel basket containing the pieces to be stripped of metal coat-ing). Workpiece 3 is suspended from a hoist 4, and attached to a suspension means 5. The workpiece 3 is electrically connected to a source of energy by electrical lead 6, hoist 4 and suspension means 5 being electrical conductors. Pot I is likewise connected to a source of electrical energy by an electrical lead 8. The current is then turned on making the pot 1 anodic and workpieces 3 cathodic. The amount of current and" voltage will depend on the bulk of the workpiece. A five pound workpiece usually requires from one hundred to three hundred amperesat about 4.5 volts. The variation in size of the workpiece would accordingly modify the required current. If the workpiece has an unusually heavy coating of metal on the base metal, the current is reversed making the pot anodic and the workpiece cathodic. This reversal of polarity may be repeated as often as desired depending on the thickness of the coated metal. After workpiece 3 is satisfactorily stripped of its metal coating, the workiece may, if desired, be allowed to drain for a few seconds, then quenched in steam preferably to rid the workpiece 3 of salt or other contaminants.

In FIGURES II and III, the structure of the system is similar to that of FIGURE I, except that anodes 9 are immersed in the system rather than using metal pot It) as the anode of the system. Workpiece 11 is immersed into molten salt composition 12, as in the systom of FIGURE I. Anodes 9 are as in FIGURE I connected to a source of direct current, workpiece 11. is also connected to a direct source of electrical energy. The electrical current is then turned on causing a stripping effect of the metal coatings on said workpiece 11. The workpieces are made cathodic, while anode means 9 are made anodic. The polarity of these electrodes may be reversed if a heavy coated metal is desired to be removed.

FIGURE III illustrates the same embodiment as FIG- URE II wherein the electrodes are reversed, the anode of FIGURE II is now thecathode, and the cathode of FIGURE II is now the anode. The same electrode reversal can be accomplished with the structure of FIG- URE I.

One of the most common sources of reclaimed copper has been copper wire used as electrical conductors, etc. These wires however have on their external portions an insulating material wrapped over a lead and/or tin coating. To reclaim the base metal copper, it is required first to remove the insulation, and subsequently remove the surface metallic coating. The following examples are illustrative of and are by no means limiting to the concept of this invention.

Example I .-Str1'pping of Lead and Tin Samples of wire for salvage were obtained and the insulation was burned off or otherwise removed. The wire was then chopped into small lengths averaging between two to four inches. These chopped short pieces were deposited into a steel basket; a source of direct current was connected to this basket. The basket was then immersed into a steel pot containing a molten eightyfive percent NaOH, five percent Na PO and ten percent KCl composition, maintained at about nine hundred degrees Fahrenheit. The pot was initially made anodic and the basket made cathodic; the current was then reversed, making the basket anodic and the pot cathodic.

A five pound batch of chopped wire requires about one hundred amperes'at five volts. The time for each cycle may .vary, but five minutes anodic followed by ten minutes cathodic proved to be preferred. The preferred method of removing the dr'agout from the workpiece in this case is by subjecting the workpiece to the action of live steam. The workpiece is now better conditioned for Water quenching, since it has, by the steam, been slowly cooled, and the chance of any violent reaction when the workpiece is water quenched, is nil. The thinly coated copper wire was stripped of lead and tin with a straight cathodic (reducing) cycle alone. On unusually difficu-lt orextra heavy coatings, a reducing-oxidizing-reducing sequence, or a series of oxidizing-reducing couplets proved to be preferred.

The salvaged copper wire was fairly bright after water quenching, but did evidence a somewhat mottled appearance with some adhering salt. For best results, the workpiece was, after treatment, contacted with a hot water soak to dissolve residual salt, followed by a bright d ip. This bright dip involved: at room. temperature, contacting the workpiece with a sulfuric-dichromate mix (three percent sodium or potassium dichromate in ten percent sulfuric acid). This converts the matted mass of heterogeneous wire into a uniform bright golden texture. The assay of the reclaimed Cu was 99.5 percent.

Example 2.-Slrz'pping of Lead and T in A five pound batch of chopped lead and tin coated copper wire was deposited in a perforated steel basket. The steel basket was immersed in a sixteen inch (16) by sixteen inch (16") by sixteen inch (16") steel electroyltic pot containing therein a composition comprising five percent Na PO ten percent KCl and eighty-five percent NaOH. The steel basket was suspended from a hoist and connected to a source of direct current (three hundred amperes selenium rectifier). The chopped wire was a mixture of sizes, diameters, and lengths, coated with lead and/or tin and/ or solder. The workpiece was made cathodic for five minutes at one hundred amperes, and 4.4 volts; then was made anodic for fiv'e minutes at one hundred amperes and 4.6 volts. The basket was raised from the salt, allowed 7 to drain for a few seconds, then quenched in cold water and rinsed free of salt in hot wa ter. There was no visible evidence of any lead or tin remaining on any of the copper wires.

Example 3.-Stripping of Lead and Tin The same conditions, batch size, basket and pot as in Example '2. The process included five minutes of oxidizing (anodic at onehundred amperes at 4.5 volts), plus ten minutes reduction (cathodic) one hundred amperes at 4.6 volts), plus water quench followed by a hot water soak. No evidence of lead or tin, but slightly mottled (non-uniform) copper color on surface.

Example 4.Stripping of Lead and Tin 'new copper resulted.

Example 5 .Stripping of Lead and Tin The same conditions-as in Example 4 wherein a five pound batch of wire was deposited in the basket and processed for fifteen minutes of straight reduction only (cathodic one hundred amperes, 4.6 volts), plus water quench, followed by a hot water soak. No evidence of lead or tin, but not particularly bright finish on the copper.

Example 6.S tripping of Lead and Tin Repeated the process of Example 5 followed by a bright dip for forty-five seconds in a sulfuric acid-dichromate composition at room' temperature, rinsed and dried. A bright, clean product resulted with no evidence of lead or tin remaining.

Example 7 .Slripping of Lead and Tin Same process as in Example 2, followed by a bright dip, forty-five seconds in sulfulric acid-dichromate mixture, at room temperature. A very clean, bright golden finish was achieved with no trace of lead or tin.

Example 8.Stripping of T in- Aone and three-quarter inch strip was cut from a tin can; the strip was six inches in diameter and was suspended from the hoist. This strip was made cathodic for two minutes at eighty amperes and 7.0 volts. It was quenched in water, followed by a hot water rinse; it was one hundred percent detinned clean steel.

Example 9.Slripping of Tin From a tin can, a strip four and three-quarter inches by six inches in diameter was suspended into the melt as it was in Example 8. It was made cathodic for two minutes at eighty amperes and 6.0 volts. The results were one hundred percent detinned clean steel.

Example 10.-Stripping of Phosphate Coating A three by five inch 1010 steel panel, used in thi'stest was coated with a light, brown to blue clear phosphate coating known as SRP-35 Iron. This panel contained from 10.2 to 16.5 milligrams of iron phosphate. Initial weight of the panel was 6115741 grams. Connection was made to a source of direct current and the panel was made negative. Sensitive ammeter and voltmeter was included in the circuit at the point of use (that is, at the salt). Salt composition was: NaOH, fifty to ninety-five percent; Na PO five to ten percent; Na SnO two to five percent; KCl, zero to ten percent; Na CO .2 to twenty-five percent. Salt temperature was nine hundred degrees Fahrenheit. The following data was obtained:

Immersion Amperes Volts Comments of Result time 1 min 2.0 1. 67 1% min 2.0 1. 72 2 min 2. 1. 72 Steady. Sample removed and quenched in water, then rinsed and dried. It was bright, clean metal.

Current density= amperes sq. ft. Final weight of panel=6L5600 gm. L0ss=0.0141 gm., or 14.1 mg.

Example 11.Stripping of Phosphate Coating Salt temperatureznine hundred degrees Fahrenheit. Current density=ten amperes per square inch.

Final weight of panel=49.3376 gm.

Loss:0.0646 gm., or 64.6 mg.

Immersion Amperes Volts Comments of Result time 30 sec 2.0 1. 55

1min 2.0 1.70

1% min 2.0 1. 74

2 min 2.0 1. 74 Steady. Sample removed and quenched in water. Rinsed and dried. The result was a bright, clean, metallic sample.

Example 12. Stripping of Oxides A panel of 1010 steel, seven-eighths of an inch by two and one-half inches, with a blued protective oxide film, was connected to the source of direct current and made the cathode in the circuit through the salt composi tion of Example 11 and to the pot wall serving as an anode. Salt temperature was nine hundred and fifty degrees Fahrenheit. Current used was ten amperes at 1.8 volts, for five minutes. This was a current density of three hundred and twenty-nine amperes per square foot. The panel was removed, quenched, washed, dried and examined. It was bright, clean, metallic iron-the blue oxide was completely stripped from the surface.

Example 13.Stripping of Oxides A panel of eighty percent nickel steel, coated with a thin, tenacious, black protective oxide, was connected as cathode in a DC. circuit using the pot as anode. The three by six inch panel was processed as in Example 12 in the same salt, ten amperes at 2.4 volts for five minutes. This was a current density of forty amperes per square foot. After quenching and drying, panel was examined and found to be brilliantly bright and metallic, with no trace of the black oxide coating.

Example 14.Stripping of Oxides A dull, black oxide-coated brass nozzle, two inches in diameter and five inches long, was hung from a hook connected to a source of direct current as cathode in a circuit through a salt composition as in Example 10 to the steel pot wall as anode. A current of ten amperes at 2.5 volts for five minutes was employed, at which point the nozzle was removed and quenched in cold water, rinsed in hot water, and dried. Current density over the whole surface was estimated to be 30.0 amperes per square foot. The nozzle exhibited its bright, golden metallic appearance, without any trace of the original black oxide coating. It was completely stripped.

Technical grade caustic soda is normally preferred since it is commercially available at a relatively low cost. Hence, when technical grade caustic soda is employed, small amounts of impurities normally associated therewith are present in the bath, i.e., around three (3) percent cumulatively by weight of sodium chloride, sodium carbonate, etc. based on the total composition. It will be noted that in use the alkali hydroxide-based molten salt bath slowly absorbs carbon dioxide from the atmosphere and converts some of the hydroxide to alkali carbonate. Another source of carbon dioxide in the proximity of the bath would be the exhausts of gas or oil-fired heating tubes for maintaining the salt in the molten state. This conversion of alkali metal hydroxide to alkali metal carbonate in no way adversely aifects the utility of the salt bath; in fact, carbonate below thirty (30) percent increases the fluidity of the salt due to a lowering of the melting point of the mixture.

Although this invention has been illustrated and defined herein in terms of the above examples and accompanying drawings, it is to be understood that these are by no means all inclusive. Various modifications to the invention herein set out will suggest themselves to those skilled in the art. These are intended to be comprehended within the spirit of this invention.

Iclaim:

1. A method of stripping a surface coating from a metallic workpiece which comprises contacting a coated workpiece with an electrolyte in an electrochemical system said electrolyte consisting essentially of a molten bath for the electrolytic stripping of surface coatings from the metal resulting from fusing together at least 50 percent of an alkali metal hydroxide with 1 to 50 percent of a substance selected from the group consisting of sulfates, stannates, zincates, plumbates, silicates, molybdates, titanates, zirconates fluotitanates fluozirconates, and mixtures thereof, and being substantially non-reactive with said workpiece.

2. The method of claim 1 wherein the base metal of the workpiece is a metal selected from the group consisting of steel, copper, copper alloys and mixtures thereof.

3. The method of claim 2 wherein said surface coating is a substance selected from the group consisting of lead, tin, phosphates, chromates, oxides, and mixtures thereof.

4. The method of claim 1 wherein the alkali metal hydroxide is sodium hydroxide 5. A method of stripping a surface coating from a metallic workpiece which comprises contacting a coated workpiece with an electrolyte in an electrochemical system, said electrolyte consisting essentially of a molten bath for the electrolytic stripping of surface coatings from metal resulting from fusing together about percent by weight of sodium hydroxide, about 5 percent by weight of trisodium phosphate, and about 10 percent by weight of potassium chloride.

6. A method of stripping a surface coating from a metallic workpiece which comprises contacting a coated workpiece with an electrolyte in an electrochemical system, said electrolyte consisting essentially of a molten bath for the electrolytic stripping of surface coatings from metal resulting from fusing together at least 50 percent of an alkali metal hydroxide with l to 50 percent of a substance selected from the group consisting of sulfates, stannates, zincates, plumbates, silicates, molybdates, titanates, zirconates, fluotitanates, fiuozirconates, and mixtures thereof, and being substantially non-reactive with said workpiece, said electrochemical system comprising in addition to said electrolyte anode means and cathode means, said anode means comprising said workpiece; and reversing the current, whereby said workpiece is made cathodic and said cathode means is made anodic.

7. The method of claim 6 whereby said anode means comprises a metal pot containing said electrolyte.

8. The method of stripping a lead coating from a copper basis which comprises contacting the lead coated copper basis with an electrolyte in an electrochemical system, said electrolyte consisting essentially of a molten bath for the electrolytic stripping of surface coatings from metal resulting from fusing together about eightyfive percent by weight of sodium hydroxide, about five percent by weight of trisodium phosphate, and about ten percent by weight of potassium chloride.

9. The method of stripping a tin coating from a copper basis which comprises contacting the tin coated copper basis with an electrolyte in an electrochemical system, said electrolyte consisting essentially of a molten bath for the electrolytic stripping of surface coatings from metal resulting from fusing together about eighty-five percent by weight of sodium hydroxide, about five percent by weight of trisodium phosphate, and about ten percent by Weight of potassium chloride.

10. A nonoxidizing composition adapted for use as a molten bath for the electrolytic stripping of surface coatings from metal which consists essentially of the product resulting from fusing together at least fifty percent of an alkali metal hydroxide with' 1 to 50 percent'of a substance selected from the group consisting of sulfates, stannates, zincates, plumbates, silicates, molybdates, titanates, zirconates, fluotitanates, fluozirconates and mixtures thereof.

11. The composition of claim 10 wherein said alkali metal hydroxide is sodium hydroxide.

12. A composition for use as a molten bath for the electrolytic ;stripping of surface coatings from metal which consists essentially of the product resulting from fusing together about 35.0 percent sodium hydroxide, about 44.0 percent potassium hydroxide, about 5.0 percent trisodium phosphate, about 5.0 percent sodium stannate, about 10.0 percent potassium chloride, and about 1.0 percent sodium carbonate.v

References Cited by the Examiner UNITED STATES PATENTS 948,681 2/ 1910 Chance 204-66 2,655,473 10/ 1953 Lowenheim 204-146 2,738,294 3/1956 Spence 204-145 2,826,539 3/1958 Murtland 204-145 2,920,023 1/1960 DAquila 204-145 2,936,270 5/ 1960 Webster et a1 204-141 2,936,278 5/ 1960 Shoemaker 204-145 3,096,261 7/1963 Mekjean 204-141 FOREIGN PATENTS 419,819 11/1934 Great Britain.

JOHN H. MACK, Primary Examiner.

P. SULLIVAN, R. GOOCH, RH. HARDER, R. K.

MIHALEK, Assistant Examiners. 

1. A METHOD OF STRIPPING A SURFACE COATING FROM A METALLIC WORKPIECE WHICH COMPRISES CONTACTING A COATED WORKPIECE WITH AN ELECTROLYTE IN AN ELECTROCHEMICAL SYSTEM SAID ELECTROLYTE CONSISTING ESSENTIALLY OF A MOLTEN BATH FOR THE ELECTROLYTIC STRIPPING OF SURFACE COATINGS FROM THE METAL RESULTING FROM FUSING TOGETHER AT LEAST 50 PERCENT OF AN ALKALI METAL HYDROXIDE WITH 1 TO 50 PERCENT OF A SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF SULFATES, STANNATES, ZINCATES, PLUMBATES, SILICATES, MOLYBDATES, TITANATES, ZIRCONATES FLUOTITANATES FLUOZIRCONATES, AND MIXTURES THEREOF, AND BEING SUBSTANTIALLY NON-REACTIVE WITH SAID WORKPIECE.
 10. A NONOXIDIZING COMPOSITION ADAPTED FOR USE AS A MOLTEN BATH FOR THE ELECTROLYTIC STRIPPING OF SURFACE COATINGS FROM METAL WHICH CONSISTS ESSENTIALLY OF THE PRODUCT RESULTING FROM FUSING TOGETHER AT LEAST FIFTY PERCENT OF AN ALKALI METAL HYDROXIDE WITH 1 TO 50 PERCENT OF A SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF SULFATES, STANNATES, ZINCATES, PLUMBATES, SILICATES, MOLYBDATES, TITANATES, ZIRCONATES, FLUOTITANATES, FLUOZIRCONATES AND MIXTURES THEREOF. 